u just need to respect his boundaries, don't command him and later encourage him (just choose the choices that makes him happy) and he'll give u the special icon (^.^)
tho I can't get nulla's special icon with hearts around it and eye open that I saw it in the CG...
TYSM!!! i got nulla's on accident and i think what you have to do is mostly be affectionate with him and not to disagree with him much. im not too sure tho so you can check in the official dc, someone prob posted a guide there